Solid state switches or relays have been used for decades for various audio applications, an example of this type of circuitry is disclosed, e.g. in U.S. Pat. No. 4,438,356. Moreover, the use of solid state relays for mute purposes is not new. It has for example been used for muting hybrid amplifiers in order to avoid excessive pop/click. A known application is in the B&O (Bang & Olufsen) product “BeoLab5” where it was used to mute the ICEpower™ amplifier to suppress pop/click.
Although a significant improvement, present prior art designs still suffer from some drawbacks. For example, one concern is that the known designs result in a minor pop/click at the speaker(s) coming from the control circuit. This pop/click is actually perceivable, which has been the case for all solid state designs done up till now.
The prior art designs often rely on intelligent SW control for timing. The power up/down signal/no signal conditions must be under SW control to avoid pop/click and even worse to avoid stressing the solid state circuit (in some cases). This is a major weakness and might result in that SW errors cause defects in the HW.
As mentioned, the different solutions known in the prior art typically suffer from a remaining pop/click which is a little too significant at least for really high quality perception. There is therefore a need for a solution that reduces the remaining pop/click to an acceptable level for residual noise in high quality products like B&O (Bang & Olufsen) equipment. The most used solution today is to break the ground connection when not BTL (Bridge Tied Load). However, there is no generic solution for the two modes.
Thus, there is furthermore a need for a solution that is generic and can be used in ground wire break, hot wire break and BTL (Bridge Tied Load) mode break.
There have been some attempts to alleviate some of the above-discussed drawbacks in solid state relays, an example of this can be found in U.S. Pat. No. 4,682,061 which discloses a MOSFET switch control. The design presented in U.S. Pat. No. 4,682,061 described a balanced control circuit in the form of two current generators that outbalance each other. The current generators are realized by means of diodes which are known to act as voltage independent current generators when applied with reverse voltage. However, a solution of this type is very sensitive to tolerances of the diodes, which inherently can be very sensitive to temperature and voltage variations. Moreover, matching diodes in order to provide matched current sources is extremely difficult and it is not uncommon that variations in leakage current can differ by a factor of 1000, thus making it more or less impossible to control the time constants which are dependent on the leakage currents. Moreover, the use of an opto-coupled transistor in order to control the time constant of the “mute” is a rather costly solution.
Accordingly, there is a need for a cost-efficient and robust circuit design that takes care of the problems mentioned above and comes up with a sort of generic solution that can also be used in other applications than the ones we see today.